Replaces a multicast delegate as an event's backing store. Duplicate behavior, except it is thread safe and holds weak references.

WeakEventSource.cs

// MIT license, copyright 2015 Joseph N. Musser II

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.CompilerServices;

namespace jnm2
{
    /// <summary>
    /// Replaces a multicast delegate as an event's backing store. Duplicate behavior, except it is thread safe and holds weak references.
    /// </summary>
    public sealed class WeakEventSource<TDelegate> where TDelegate : class
    {
        // ReSharper disable once StaticMemberInGenericType
        private static readonly DynamicMethod raiseMethod;

        private readonly ConditionalWeakTable<object, List<object>> keepAliveDelegatesWithTarget = new ConditionalWeakTable<object, List<object>>();

        static WeakEventSource()
        {
            if (!typeof(TDelegate).IsSubclassOf(typeof(Delegate))) throw new InvalidOperationException("TDelegate must derive from System.Delegate.");

            
            // Cache raise method per delegate type
            var raiseSignature = typeof(TDelegate).GetMethod("Invoke");

            var parameters = raiseSignature.GetParameters();
            var raiseParameterTypes = new Type[parameters.Length + 1];
            raiseParameterTypes[0] = typeof(WeakEventSource<TDelegate>); // First parameter is raise delegate target (this)
            for (var i = 0; i < parameters.Length; i++)
                raiseParameterTypes[i + 1] = parameters[i].ParameterType;

            raiseMethod = new DynamicMethod("Raise",  raiseSignature.ReturnType, raiseParameterTypes, typeof(WeakEventSource<TDelegate>));
            var il = raiseMethod.GetILGenerator();
            
            il.DeclareLocal(typeof(List<TDelegate>)); // loc_0
            il.DeclareLocal(typeof(int)); // loc_1
            if (raiseSignature.ReturnType != typeof(void)) 
            {
                il.DeclareLocal(raiseSignature.ReturnType); // loc_2
                // var r = default(TReturn)
                if (raiseSignature.ReturnType.IsClass)
                {
                    il.Emit(OpCodes.Ldnull);
                    il.Emit(OpCodes.Stloc_2);
                }
                else
                {
                    il.Emit(OpCodes.Ldloca_S, 2);
                    il.Emit(OpCodes.Initobj, raiseSignature.ReturnType);
                }
            }

            // var handlerList = this.GetLiveHandlersReverseOrder();
            il.Emit(OpCodes.Ldarg_0);
            il.Emit(OpCodes.Call, typeof(WeakEventSource<TDelegate>).GetMethod("GetLiveHandlersReverseOrder", BindingFlags.Instance | BindingFlags.NonPublic));
            il.Emit(OpCodes.Stloc_0);

            // var i = handlerList.Count - 1;
            il.Emit(OpCodes.Ldloc_0);
            il.Emit(OpCodes.Callvirt, typeof(List<TDelegate>).GetMethod("get_Count"));
            il.Emit(OpCodes.Ldc_I4_1);
            il.Emit(OpCodes.Sub);
            il.Emit(OpCodes.Stloc_1);

            var loopConditionLabel = il.DefineLabel();
            var loopBodyLabel = il.DefineLabel();
            il.Emit(OpCodes.Br_S, loopConditionLabel);
            il.MarkLabel(loopBodyLabel);

            // handlerList[i]
            il.Emit(OpCodes.Ldloc_0);
            il.Emit(OpCodes.Ldloc_1);
            il.Emit(OpCodes.Callvirt, typeof(List<TDelegate>).GetMethod("get_Item"));

            // .Invoke(p1, p2, p3, ...)
            for (var parameterIndex = 1; parameterIndex < raiseParameterTypes.Length; parameterIndex++)
                il.Emit(raiseParameterTypes[parameterIndex].IsByRef ? OpCodes.Ldarga_S : OpCodes.Ldarg_S, parameterIndex);
            il.Emit(OpCodes.Callvirt, typeof(TDelegate).GetMethod("Invoke"));
            if (raiseSignature.ReturnType != typeof(void)) il.Emit(OpCodes.Stloc_2);

            // i--;
            il.Emit(OpCodes.Ldloc_1);
            il.Emit(OpCodes.Ldc_I4_1);
            il.Emit(OpCodes.Sub);
            il.Emit(OpCodes.Stloc_1);

            il.MarkLabel(loopConditionLabel);
            // i >= 0;
            il.Emit(OpCodes.Ldloc_1);
            il.Emit(OpCodes.Ldc_I4_0);
            il.Emit(OpCodes.Bge_S, loopBodyLabel);

            if (raiseSignature.ReturnType != typeof(void)) il.Emit(OpCodes.Ldloc_2);
            il.Emit(OpCodes.Ret);
        }


        

        public WeakEventSource()
        {
            this.Raise = (TDelegate)(object)raiseMethod.CreateDelegate(typeof(TDelegate), this);
        }

        public TDelegate Raise { get; private set; }


        
        private readonly List<WeakReference<TDelegate>> handlers = new List<WeakReference<TDelegate>>();

        public void Subscribe(TDelegate addend)
        {
            if (addend == null) return;

            lock (handlers)
            {
                handlers.Add(new WeakReference<TDelegate>(addend));
                // Ensure that the addend stays alive as long as the target
                keepAliveDelegatesWithTarget.GetOrCreateValue(((Delegate)(object)addend).Target).Add(addend);
            }
        }
        public void Unsubscribe(TDelegate subtrahend)
        {
            if (subtrahend == null) return;

            lock (handlers)
            {
                for (var i = handlers.Count - 1; i >= 0; i--)
                {
                    TDelegate del;
                    if (!handlers[i].TryGetTarget(out del))
                    {
                        handlers.RemoveAt(i); // May as well clean up the list while we search, since we have to call TryGetTarget anyway
                        continue; 
                    }
                    
                    // Once we've removed the subtrahend exactly once, return.
                    if (del.Equals(subtrahend))
                    {
                        List<object> delegates;
                        if (keepAliveDelegatesWithTarget.TryGetValue(((Delegate)(object)subtrahend).Target, out delegates)) delegates.Remove(subtrahend);
                        handlers.RemoveAt(i);
                        return;
                    }
                }
            }
        }

        
        // Retrieve the current list of live handlers to be invoked.
        // (NB! Never invoke a delegate while holding a lock; it raises the chance of contention and often causes reentrancy errors in hard-to-predict places of the host program.)
        // ReSharper disable once UnusedMember.Local
        private List<TDelegate> GetLiveHandlersReverseOrder()
        {
            var r = new List<TDelegate>(handlers.Count);

            lock (handlers)
            {
                for (var i = handlers.Count - 1; i >= 0; i--)
                {
                    TDelegate del;
                    if (handlers[i].TryGetTarget(out del))
                        r.Add(del);
                    else
                        handlers.RemoveAt(i);
                }
            }

            return r;
        }
    }
}

Not sure if you saw my reply on my blog, so I repost it here:

I just had a look at your solution. Actually, keeping a weak reference to the delegate itself is something I tried before, but it has a serious drawback: the subscriber needs to keep a strong reference to the delegate, otherwise the delegate is garbage collected too early!

Here’s a snippet that demonstrates the issue:
https://gist.github.com/thomaslevesque/fc8e141f377967c0fcfe

As you can see, the Subscriber.OnFoo method is never invoked, because the delegate has already been collected.

That is an excellent point. I was mistaken- delegates are removed by value, not reference. So my code oversimplifies. I updated my code to resolve this issue using a ConditionalWeakTable to guarantee that as long as the target is alive, the delegate will stay alive. If the target does not stay alive, the delegate gets collected.

I want to be sure I understand the way MulticastDelegate.Remove works for multicast delegates.

How do you like the Raise pseudomethod?

And here is a version that introduces WeakDelegate<TDelegate> and does not need ConditionalWeakTable.

Hi Joseph,

Using a ConditionalWeakTable is a great idea! I think I would use it a bit differently, though; I don't think it's really necessary to have a list of WeakReference in addition to the CWT.

I want to be sure I understand the way MulticastDelegate.Remove works for multicast delegates.

Not quite; a delegate is immutable, and the Remove method doesn't actually affect the delegate instance on which it's called. It creates a new MulticastDelegate without the removed delegate. (similarly to String.Replace)

How do you like the Raise pseudomethod?

It's a good approach; actually I think it's the only way to expose a Raise "method" with the correct signature, since the signature isn't known in advance.

You can't enumerate a ConditionalWeakTable, so I couldn't think of anything more efficient than a separate list of weak references. What do you think of the WeakDelegate version?

Also did I explain well enough what can happen if you invoke a delegate within a lock? Since locks are reentrant by the same thread, you lose thread safety features. If one of the handlers happens to cause a call to Subscribe or Unsubscribe (not uncommon), complicated bad stuff happens. I kept all list operations atomic by making a temporary list and then invoking from that.

Not quite; a delegate is immutable...

Understood. It still would be good to mimic the add/remove behavior. It looks like multicast delegate math is not commutative; am I right that A + B - (A + B) = (A + B)? If I'm right, this means we don't have to do anything special since our subscribe/unsubscribe works that way as well.

Anyway, feel free to copy/take anything you like.